Second-harmonic generating properties of polar noncentrosymmetric aluminoborate solid solutions, Al _5-xGa _xBO ₉ (0.0 ≤ x ≤ 0.5)

A series of solid solutions of polar aluminoborate materials, Al _5-xGa _xBO ₉ (0.0 ≤ x ≤ 0.5) have been synthesized by standard solid-state reactions using Al ₂O ₃, Ga ₂O ₅, and B(OH) ₃ as reagents. The phase purities, crystal structures, and solid solution behavior of the reported materials have been investigated by powder X-ray diffraction. Solid solutions of Al _5-xGa _xBO ₉ crystallize in the polar noncentrosymmetric space group, Cmc2 ₁, with a three-dimensional structure consisting of distorted MO ₄, MO ₅, MO ₆, and BO ₃ polyhedra (M = Al or Ga). Powder second-harmonic generating (SHG) measurements on the Al ₅BO ₉ using 1064 nm radiation, indicate the material has a SHG efficiency of approximately 2 times that of α-SiO ₂ and is not phase-matchable (type 1). Further nonlinear optical (NLO) measurements on the Al _5-xGa _xBO ₉ solid solutions indicate a sharp increase in SHG efficiency up to 10 times that of α-SiO ₂ for x ≥ 0.4. Close structural examination suggests that the alignment of the asymmetric π-delocalization of BO ₃ groups is responsible for the abrupt increase of SHG efficiency.